NANO STRUCTURED BAINITE AND ITS APPLICATION

Prepared By : SONU DAS 11/MME/30

7TH SEMNational institute of technology , Durgapur

BAINITE : Bainite is a non lamellar mixture of ferrite and cementite in steel.

Formed when steel is cooled rapidly enough from Austenitizing temperature by passing the nose of the c-curve and kept isothermally at constant temperature .

Formed at a temperature range (250-550) °c.

Based on the quenching temperature it is devided into two

1. Upper Bainite ( formed between 550-350 °c , and has a morphology of feathery shaped ferrite).

2. Lower Bainite ( formed between temperature below 350 °c , and has a morphology of plate-like structure ).

Different transformed products of Austenite

Schematic illustration of Bainitic transformation

Fig 1. Fig 2.

Transformation mechanism

ferriteferrite

cementitecementite

Comparison of Austempered and Martempered steel

Nano structure of Bainite

Process of formation:- Austenitization of steel at ~950 °c followed by transfer into an oven at the low temperature say ~200 ° c to be held there for 10 days or so.

No rapid cooling is needed ,residual stresses are avoided.

With careful alloy design, Bainite transformation can be made possible at temperatures as low as 125 ° c.

The sample must be large enough because the time taken to reach 200 °c from the Austenization temperature is much less than that required to initiate Bainite.

Some amount of residual Austenite is important because it enhances the work hardening capacity of material.

The resulting microstructure consists of mixture of thin platelets of Bainitic ferrite just 20-40nm in thickness.

The high silicon content (~1.5 wt% and above) has allowed for the suppression of cementite .

Such microstructures(almost negligible)cementitecan exhibit excellent combinations of strength and toughness (1600-1700MPa, 130 MPa.m1/2).

Properties :

Achieve strength in excess of 2.5 Gpa.

Toughness ~30 Mpa.m1/2

The Bainite obtained by low temperature transformation is harder then ever achieved and will have better wear resistance.

Comparison of data for Bainitic steel

Steel Sample

Temp.

Bainite lathe thickness/nm

Retained ausetnite/%

Hardness/hv

Ys/mpa Uts/mpa El./%

0.8 c 220°c

35 16 710 1865 2444 4.9

0.8 c 250°c

37 18 659 1810 2345 6.6

0.8 c 270°c

24 24 615 1658 2226 10.2

1CSi 220°c

28 36 650 1626 2488 8.1

1CSi 250°c

28 34 625 1642 2417 17.2

Yield strength were found to correlate well with the microstructure.

By Mack & Bartlett :

o For a thickness ‘t’ of the ferrite plates the mean intercept L=2t

o By Langford and Cohen Strength is given by =115/L Mpa , where ‘L’ is in micrometer.

Weakness:- It cannot be satisfactorily welded. Restricted applications. Applications : Different military industries such as

armor. Car manufacturing such as shaft ,

bearings. Railway industries.

Thank you